Dear Colleagues,

In the last two decades, statistical analyses of astrophysical and cosmological datasets have favored the concordance ΛCDM cosmological model. In this model, the most important energy density component is dark energy (DE), needed to explain the undergoing period of accelerated expansion. The second in importance is dark matter (DM), needed to explain the emergence of large-scale structures. Nevertheless, the model lacks observational evidence of DM and DE at the particle level: Their dynamical effects are evident at galactic, extragalactic, and cosmological scales, but their fundamental nature, whether in particles or scalar fields, is still completely unknown. 

The standard cold dark matter (CDM) paradigm shows several difficulties at galactic scales. For example, the Navarro–Frenk–White density profile cannot easily justify the existence of core in low surface brightness, dwarf and ultra-faint galaxies (the so-called cuspy-core problem), and there is a discrepancy in the number of observed and predicted satellites around massive galaxies (the so-called missing satellite problem). These are only two among many other problems that CDM encounters at small scales. This “small-scale crisis” may open up new avenues to explore to solve these issues (for instance, a change of paradigm from CDM to self-interacting, warm or fuzzy DM, among others). On this side, the role of N-body simulations is strategic to understand the impact of baryonic feedback, and whether it can represent a solution to the small-scale issues within the CDM. 

Although the ΛCDM model successfully explains observations ranging from cosmic microwave background radiation to the large-scale structure of the universe, it leads to tensions on cosmological scales between high- and low-redshift determination of the cosmological parameters. An example is the well-known tension in the Hubble constant parameter. The solution of this and other issues may reside in a different paradigm of DE such as an early DE component, among others. 

The aim of this Special Issue is to discuss the concordance ΛCDM cosmological model through the problems it encounters at both cosmological and galactic scales, and the possible solutions to unveil the nature of DM and DE, and whether general relativity holds on all scales. The contributions to this issue may be related to alternative paradigms, detection strategies, N-body simulations and data analysis methods, recent discovery confirming or questioning the standard paradigm, and reviews on the state-of-the-art of the field.

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• The nature of dark matter and dark energy
• Current and future cosmological and astrophysical observations
• Current and future detection strategies for dark matter and dark energy
• Small-scale problems of CDM
• Cosmological tensions of ΛCDM
• Alternative model of DM: warm dark matter, self-interacting dark matter, fuzzy dark matter, axions, and modified gravity, among others
• Alternative model of DE: quintessence, k-essence, interacting dark energy, early dark energy, variation of fundamental constants, and modified gravity, among others
• Data analysis methods
• N-body simulations